Titanium dioxide pigments are produced by the sulfate process and by the ch-oride process.
The sulfate process is based on the digestion of titanium-containing raw materials with sulfuric acid. Titanium dioxide hydrate is precipitated from the solution formed by hydrolysis. After removal of impurities, the hydrolyzate is calcined and ground. The calcination step is carried out primarily in rotary kilns. Heating of the rotary kilns is by fuel oil or natural gas and involves high energy consumption.
In the chloride process, titanium tetrachloride is obtained from titanium-containing raw materials by chlorination under reducing conditions at 700.degree. to 1200.degree. C. Petroleum coke is used as the reducing agent.
After the separation of chlorides formed from impurities in the starting materials, titanium tetrachloride is burnt at temperatures of 900.degree. to 1400.degree. C. to form titanium dioxide pigment.
The titanium dioxide pigments obtained by the sulfate or ch-oride process are generally subjected to an inorganic and/or organic aftertreatment.
In the inorganic aftertreatment, oxides, oxide hydrates, silicates and/or phosphates are deposited, generally from aqueous solution, onto the titanium dioxide pigment suspended in water.
The titanium dioxide pigments are then filtered off, dried and micronized. Before micronization or even before drying, the titanium dioxide pigments are general-y subjected to an organic aftertreatment with dispersants, such as silicones, amines, phosphoric acid esters and organic acids.
The drying of the aftertreated titanium dioxide pigments is advantageously carried out in spray dryers or spin-flash dryers, the energy required generally being applied by combustion of natural gas or fuel oil.
In the production of titanium dioxide pigments by the chloride process, reaction gases which, in addition to titanium tetrachloride, contain dust-fine particles of the solid raw materials, chlorides of the secondary constituents of the titanium-containing raw materials and, above all, carbon monoxide and carbon dioxide accumulate during the chlorination of the titanium-containing raw materials. The carbon monoxide content of the reaction gases is generally 40 to 80%. In addition, the reaction gases may contain small quantities of chlorine, silicon tetrachloride, phosgene, hydrogen chloride, sulfur chlorides and carbon disulfide.
After separation of the dusts and the substantially involatile metal chlorides and after condensation of the titanium tetrachloride by cooling of the gases to temperatures near or below 0.degree. C., an offgas mixture is obtained which, in addition to large amounts of carbon monoxide, also contains small quantities, of chlorine, titanium tetrachloride and other readily volatile compounds. Chlorine and titanium tetrachloride are genera-ly removed by washing, after which the offgas still contains traces of chlorine and metal chlorides.
On account of its high carbon monoxide content, however, the offgas cannot be direct-y re-eased into the atmosphere, instead the carbon monoxide has to be burnt before emission. This is normally done with the assistance of a supporting flame of natural gas or fuel oil.
The object of the present invention is to provide a particular-y advantageous process for the production of titanium dioxide pigments. More particularly, the object of the invention is to reduce the high natural gas and fuel oil demand in the production of titanium dioxide pigments and hence to improve the economy of production.